The invention relates to a process for preparing carboxylic esters by reacting carboxylic acid or a carboxylic anhydride or a mixture thereof with an alcohol.
Esters of phthalic acid, adipic acid, sebacic acid or maleic acid are widely employed in surface coating resins, as constituents of paints and in particular as plasticizers for plastics.
It is known that carboxylic esters can be prepared by reacting carboxylic acids with alcohols. This reaction can be carried out autocatalytically or catalytically, for example in the presence of Brönsted or Lewis acids as catalysts. Regardless of the type of catalysis, there is always a temperature-dependent equilibrium between the starting materials (carboxylic acid and alcohol) and the products (esters and water).
The reaction of internal carboxylic anhydrides with alcohols proceeds in two steps: the alcoholysis of the anhydride to form the monoester generally proceeds rapidly and to completion. The further conversion of the monoester into the diester with formation of water of reaction is reversible and proceeds slowly. This second step is the rate-determining step of the reaction.
To shift the equilibrium in the direction of the ester (or the full ester in the case of polybasic acids), it is usual to use an entrainer by means of which the water of reaction is removed from the mixture. If one of the starting materials (alcohol or carboxylic acid) has a boiling point lower than that of the ester formed and forms a miscibility gap with water, a starting material can be used as entrainer and be recirculated to the mixture after water has been separated off. In the esterification of higher aliphatic carboxylic acids, aromatic carboxylic acids or dibasic or polybasic carboxylic acids, the alcohol used is generally the entrainer.
EP-A 1 186 593 describes a process for preparing carboxylic esters by reacting dicarboxylic or polycarboxylic acids or anhydrides thereof with alcohols, with the water of reaction being removed by azeotropic distillation with the alcohol. The amount of liquid removed from the reaction by the azeotropic distillation is replaced either completely or partly by the alcohol.
If the alcohol used serves as entrainer, it is usual to condense at least part of the vapor from the reactor, separate the condensate into an aqueous phase and an organic phase comprising essentially the alcohol used for the esterification and recirculate at least part of the organic phase to the reactor. However, various by-products are formed in addition to the desired ester in the esterification reaction. Particularly the by-products having lower boiling points than that of the alcohol are recirculated with the organic phase to the reactor and can, particularly in the case of a continuous process, accumulate in the reaction system.
It has been found that the by-products having boiling points lower than that of the alcohol comprise mainly olefins which have been formed by elimination of water from the alcohol used. Relatively high concentrations of the olefins can damage the esterification catalyst used and/or impair the quality of the ester product produced, in particular lead to undesirable discoloration. To avoid accumulation of the by-products having boiling points lower than that of the alcohol in the reaction system, it is advisable not to recirculate all of the organic phase to the reactor but rather to discharge a substream (purge stream). However, a considerable proportion of the alcohol used is lost to the esterification reaction via the purge stream.
It is therefore an object of the invention to minimize the alcohol losses via the purge stream.